As a detector for a liquid chromatograph (LC), an optical analyzer is frequently used, such as an ultraviolet-visible spectrophotometer or photodiode array detector. In recent years, with the advancement and rapid spread of light-emitting diode (LED) technology, LEDs have also been used as the light source for optical analyzers. Since LEDs have a comparatively narrow peak width in its emission spectrum, they are less suitable for applications in which a wavelength scan is performed over a wide range of wavelengths. However, they can be suitably used in an optical analyzer which irradiates a sample with a specific wavelength of light, such as an absorptiometer or fluorometer. Compared to various types of conventionally and generally used light sources, LEDs have the advantage of being dramatically inexpensive while at the same time having a long life and being highly reliable.
FIG. 7 shows a schematic configuration of an absorptiometer using an LED as the light source (for example, see Patent Literature 1).
Measurement light emitted from the LED (e.g. deep ultraviolet LED) 71 serving as the light source is cast into a flow cell 72. While passing through a sample solution in the flow cell 72, the measurement light undergoes absorption in a manner that depends on the kind and amount of a component in the sample solution. The light which has undergone such an absorption enters a photodetector 73. The photodetector 73 produces a detection signal corresponding to the amount of that light. In a signal processing unit (not shown), the absorbance by the sample is calculated from the detection signal.
In order to enhance the speed and sensitivity of an analysis in an LC using such an absorptiometer as the detector, it is essential to reduce the dispersion of the sample (spread of the peak) within a passage other than the column. To this end, low-volume flow cells have been in demand. To meet such demand for low-volume cells, Patent Literature 2 discloses a technique in which a flow cell made of a silicon oxide or similar material is created by using a semiconductor manufacturing process. The use of the microfabrication technique employing the semiconductor manufacturing process enables the creation of a low-volume flow cell with a high level of dimensional precision.
Such a low-volume flow cell is intrinsically small and lightweight, thereby being convenient for reducing the size and weight of the absorptiometer. However, for high-sensitivity and high-accuracy measurements in an absorptiometer, it is necessary to manually align the optical axes of the light source and the photodetector in the process of device assembly so as to make the beam axis of the measurement light pass through a predetermined position within the flow cell. The smaller the flow cell is, the more difficult this aligning task becomes. Therefore, the device assembly requires not only a considerable amount of time and labor but also workers skilled in the assembling task. Furthermore, even if the flow cell is miniaturized, there is a limit on the size reduction of the entire device due to restrictions on the sizes of the parts in the light source and photodetector.